fix: use created_at for message timestamps to prevent Invalid date on history messages

[NaitikVerma6776]

🔗 Related Issue

Closes #728

---

📝 What does this PR do?

MessageBubble.tsx was extracting the display timestamp by splitting the message ID on "-" and parsing the second segment as a number:

new Date(Number(message.id.split("-")[1]))

This works for streamed messages, which use synthetic IDs like user-1717000000000. But all messages loaded from chat history use real database UUIDs (e.g. 550e8400-e29b-41d4-a716-446655440000).

Splitting a UUID on "-" and parsing "e29b" as a Number gives NaN → new Date(NaN) → formatDistanceToNow renders "Invalid date" under every historical message.

Fix:

• Added a getMessageDate() helper that first tries
  message.created_at (returned by the history API for
  all persisted messages), falls back to the synthetic
  ID-based timestamp for streamed messages, and returns
  null if both produce an invalid date
• Timestamp div only renders when messageDate is not
  null — no more "Invalid date" shown to users
• Added created_at?: string to the ChatMsg interface
  in chat-store.ts so TypeScript correctly types the
  field the history API already returns

---

🗂️ Type of Change

• 🐛 Bug fix

---

🧪 How was this tested?

• Ran the frontend locally (npm run dev inside frontend/)
• Sent a message → confirmed timestamp shows correctly
  during streaming ✅
• Refreshed the page → reloaded the same chat session
  from history → confirmed timestamps now show correctly
  (e.g. "2 hours ago") instead of "Invalid date" ✅
• Confirmed response_time_ms badge still renders
  correctly alongside the timestamp ✅
• Ran npx prettier --write on both changed files ✅
• TypeScript build passes ✅

---

⚠️ Anything to flag for reviewers?

Two files changed:

1. frontend/src/components/chat/MessageBubble.tsx — core fix: getMessageDate() helper + updated
   timestamp render
2. frontend/src/store/chat-store.ts — one line added: created_at?: string on ChatMsg interface to properly type the field the history API already returns (previously untyped/missing from the interface)

No other behavior is affected — streamed messages continue to work exactly as before via the ID-based fallback.

---

✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style
  (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[vercel]

@NaitikVerma6776 is attempting to deploy a commit to the param20h's projects Team on Vercel.

A member of the Team first needs to authorize it.